1. Field of the Invention
The present invention relates to a broadband wireless access communication system, and more particularly to a method for reducing power consumption of a user terminal when synchronization between the user terminal and a base station is lost because of a poor channel environment in a broadband wireless access communication system such as IEEE 802.16e.
2. Description of the Related Art
With the 4th generation (hereinafter, referred to as “4G”) communication system, a next generation communication system, providing users with services having various qualities of service (hereinafter, referred to as “QoS”) and transmission speeds of approximately 10 Mbps are the focus of the developers.
At present, the 3rd generation (hereinafter, referred to as “3G”) communication system supports a transmission speed of approximately 384 kbps in an outdoor channel environment, a relatively inferior channel environment, and supports a transmission speed of approximately 2 Mbps in an indoor channel environment, a relatively superior channel environment. A wireless local area network (hereinafter, referred to as “LAN”) and a wireless metropolitan area network (hereinafter, referred to as “MAN”) generally support transmission speeds of approximately 20 Mbps to 50 Mbps.
In the 4G communication system, new communication techniques are under development, which attempt to provide mobility and high QoS to the user terminal in the wireless LAN and MAN systems while attempting securing a relatively high transmission speed. Studies are also actively progressing to support a high-speed communication service, in the 4G communication system.
Since a wireless MAN system provides a wide service coverage area and supports high speed transmission, the wireless MAN system is suitable for supplying a high-speed communication service. However, the wireless MAN system does not provide for mobility of the user terminals and therefore does not provide a handoff procedure to control high-speed movement of a user terminal.
The IEEE 802.16e standard adds techniques to compensate for the mobility of a user terminal in a MAN which has a high-speed transmission function over a range of up to 75 km, while including a LAN capabilities. The IEEE 802.16e standard has been developed to provide data service, video service, voice service, etc., with a prescribed QoS in a point-to-multipoint network structure. According to such a standard, it is possible to provide broadband service on the assumption that the user terminal is fixed in its location. The IEEE 802.16e standard aims at enlarging a service area of the user terminal comparable to that of a cellular service environment by enabling the user terminal to receive services even while moving and by providing a handoff function to the user terminal on the basis of the IEEE 802.16a standard. The IEEE 802.16e standard includes link budget improvement, a plan to allow communication during a high speed movement of the user terminal, addition of a handoff function, addition of a low-power mode, etc.
According to the IEEE 802.16e, a modulation method is not fixed to any one method, but can be varied according to a channel environment. Modulation methods which can be used in the IEEE 802.16e include quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (16QAM), 64 quadrature amplitude modulation (64QAM) methods. Such modulation methods are applied as needed to adapt to a channel environment and a cell coverage range.
A base station and a user terminal must be synchronized with each other to provide communication between them according to the IEEE 802.16e. Although synchronization between the base station and the user terminal is established, the synchronization between them is frequently disrupted disconnected due to a poor channel state. The IEEE 802.16e standard does not define operations which are expected to be performed by the base station and the user terminal after synchronization between the base station and the user terminal is disrupted.
When the synchronization between the base station and the user terminal is disrupted the user terminal continues to maintain a state in which it was operating just prior to losing synchronization. Although the user terminal maintains such an operating state, it is impossible to perform communication until synchronization between the base station and the user terminal has been reestablished. Although synchronization between the base station and the user terminal is disconnected, the user terminal continues to perform full, high power operations which it was performing before the loss of synchronization, so that unnecessary power is continuously discharged.
Therefore, a technique for reducing power in a user terminal caused by unnecessary power discharge in a state in which synchronization between the user terminal and the base station is disconnected is needed. Furthermore, the continued discharge of power by the user terminal during times when synchronization is disconnected, unnecessary interference may occur between the relevant channel and an adjacent channel.